Hi Stefan!
I wasn't following the discussion too closely yet but I had the best
experience with pre-amplifiers when the very first stage is "passive"
and not an active device as in your schematic.
So in my opinion, you should consider to put a passive (L-C)
bandpass-filter right before the signal reaches the first active device.
Most of the distortion and other unwanted mixing is generated by the
first active device.
73
OE3GHB
Gerhard
Am Donnerstag, den 05.08.2010, 20:06 +0200 schrieb Stefan Schäfer:
> Thanks for all answers so far.
>
> Alan, i think that filtering is essential for me since DLF (153 kHz,
> 500kW) is just 40km distanced. I am afraid that the amplifier gets
> overdriven else. For DCF39, which is now almost not filtered (maybe
> 10...20 dB) it is the same. This level is 60 dB above noise and i need
> some 20 dB more gain after the BF981 since the level is still to low for
> the soundcard. SpecLab can do a lot, clear, but what if the amplifier
> becomes nonlinear due to those high levels? This is my worry. In UK, DLF
> or DCF39 is not that issue i assume (?) ;-)
>
> What is the problem with rapid phase changes? I am no communication
> engineer, sri, but i like to learn on that! (remembering that before
> some months i asked what is the "gain" when going from qrss3 to qrss30
> ;-) ).
>
> Michels idea sounds good. In principal, this is the same what i have
> done before the mixer. Applying 2 further band filters means parts
> effort but this doesn't matter really. If i put one filter to 12,7 kHz
> (QRSS3) and the other to say 11,6 kHz (CW and european transmit window)
> this would work, regarding the attenuation of DCF39. And i can adjust
> the filter characteristic directly by watching the SpecLab window and a
> broad band noise source (my soldering station does a good job there when
> placing the antenna near it :-) ).
>
> I will keep your suggestions in mind. Maybe a call to Markus can
> convince my completely ;-)
>
> 73, Stefan
>
>
>
> Am 05.08.2010 17:00, schrieb Alan Melia:
> > Hi Stephan....why do you think you need a narrow filter at 12kHz?? Why not
> > let the sound card sort it out? Provided you have killed the image (113kHz)
> > there should not be a problem. If you use a narrow passive filter you risk
> > rapid phase changes near the wanted frequency. This is probably not a good
> > idea. I suspect that a fairly "benign" low pass filter (Butterworth??) just
> > above 12Khz (to aid the anti-alias filtering) and another Butterworth to
> > remove any 50Hz and low harmonics of that say below 1kHz. this leaves a
> > fairly flat pass-band with a slowly changing phase response.
> >
> > What may be more important may be getting a good low noise amp to feed the
> > sound-card. It is worth a look at some of the softrock workand circuits
> > here. Also Paul did some work on this some time back. Jim may have some more
> > helpful ideas in this area. I have not found conventional filtering in front
> > of an FFT does a lot of good and it certainly has the potential to "muddy"
> > things up.
> >
> > Alan G3NYK
> >
> > ----- Original Message -----
> > From: "Stefan Schäfer"<[email protected]>
> > To:<[email protected]>
> > Sent: Thursday, August 05, 2010 2:32 PM
> > Subject: LF: Bandpass filter design
> >
> >
> >
> >> Dear LF,
> >>
> >> Currently i am setting up my active antenna for the planned LF grabber
> >> here in Heidelberg. It is an active E field antenna, using a BF981 and a
> >> 125 kHz signal that transforms the 137 kHz down to 12 kHz where some
> >> band filtering has to be applied. Then, i need another amp stage to
> >> drive the soundcards input (BF862). The high impedance of the wire input
> >> is first down transformed by a BF862 stage as a source follower, then i
> >> allpy a double LF bandpassfilter that is coupled by a C of some pF
> >> (about 4...8 pF). This signal is applied to the 2nd Gate of the BF981...
> >>
> >> My question is: There may be better suited filter designs than taking a
> >> L parallel C resonated at 12 kHz (after the mixing stage), between
> >> signal and ground since this gives a sharp filter, ie 137,7 kHz is
> >> already attenuated by 25 dB compared to 137,0 kHz. What i want to have
> >> is a filter with a specific bandwith and edge frequencies with about
> >> constant low attenuation in the transmission range and relative sharp
> >> slopes so that 137,7 kHz is not really attenuated but 138,83 kHz
> >> (DCF-39) as much as possible. DCF39 is 60 dB above noise here although
> >> it gets already attenuated by the input band filter!
> >>
> >> Jim/M0BMU has designed a filter for his VLF loop RX that looks quite
> >> good. Is there a web page where i just can type the filter oder, edge
> >> frequencies, input- output impedances and so on and get the values?
> >> I have found such one at
> >> http://www-users.cs.york.ac.uk/~fisher/cgi-bin/lcfilter but i am not yet
> >> experienced too much about this stuff so i am not sure if this gives
> >> really useful answers.
> >>
> >> Any other simple ideas to come quickly to the optimal filter type, oder
> >> and values? I do not want to spend too much time for that, so an
> >> "excellent filter design book" is not the best hint ;-)
> >>
> >> What about a cauer filter? I have read that it has the sharpest edges
> >> but this may cause QRM in the pass band? (like clicks in a too sharp CW
> >> filter?)
> >>
> >> The picture shows what i have done so far. Watching the spectrum on the
> >> roof of the institute (the future QTH) from 0...48 kHz in SpecLab looks
> >> very promising so far (see picture). DLF is 60 dB above noise although
> >> already attenuated about 60 dB! So filtering before mixing and further
> >> amplification is necessary in my case, i assume...
> >>
> >> Tnx for helping ideas.
> >>
> >> 73, Stefan/DK7FC
> >>
> >>
> >
>
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